1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly, to field terminating structures for improving the breakdown voltage of high voltage semiconductor devices.
Semiconductor devices such as diodes, transistors, thyristors, integrated circuits and the like generally have one or more p-n junctions formed in a substrate of semiconductor material. A p-n junction is a boundary between a region of p-type material and a region of n-type material. These regions are typically formed by diffusing impurities into the substrate. The impurities generally diffuse laterally as well as vertically into the substrate. In order to increase the packing density of the various elements on the substrate it is necessary to minimize the amount of lateral diffusion. This accomplished by making the diffusions relatively shallow.
Electric fields are inherently generated at the p-n junction, with the intensity of the field at a particular location being related to the curvature of the diffusion. Shallow diffusions have a relatively small radius of curvature at the edges of the diffusion, which accentuates the electric field. A reverse biasing voltage applied across the p-n junction further increases the electric field. If the intensity of the electric field becomes too great at any particular point, the p-n junction can break down. The voltage at which a particular p-n junction breaks down is typically referred to as the breakdown voltage. Because of the relatively small radius of curvature at the edges of shallow diffusions, shallow diffusions generally have a lower breakdown voltage than deeper diffusions. Thus, while it is desirable to utilize shallow diffusions so as to increase the packing density of elements such as transistors on a chip, the use of such diffusions brings about the undesirable result of reducing breakdown voltage.
2. Description of the Prior Art
Several types of structures have been utilized to improve the breakdown voltage of a p-n junction. One technique is the use of one or more guard rings diffused into the semiconductor material (FIG. 1). The guard rings, also called floating field rings, are intended to drop the applied voltage along the surface of the material to gradually reduce the intensity of the electric field in the area of the junction curvature. One disadvantage of the guard rings is that the actual voltage gradient obtained critically depends upon the spacing of the guard rings. Furthermore, the guard rings do not resolve the problem of mobile charge carriers in the insulating oxide layer and on the surface of the semiconductor material. These carriers can be attracted by the potential applied to one of the electrodes of the junction, which can adversely affect the shape of the depletion region of the junction close to the surface, with a resulting increase in the electric field magnitude. Furthermore, these carriers can cause an inversion layer on one side of the junction such that if the inversion layer reaches the electrode, current may flow directly, bypassing the p-n junction, and enormously increasing leakage current.
Another technique for increasing the breakdown voltage of shallow diffused junctions is the junction field plate (FIG. 2) which generally is a metal plate overlapping the p-n junction on an insulating layer of silicon dioxide. The field plate is added to a device to deplete the surface of the semiconductor material of charge carriers in the vicinity of the junction edge so as to reduce the electric field and thereby increase the breakdown voltage. Also, the field plate is intended to prevent the formation of an inversion layer bypassing the p-n junction by attracting mobile ions toward the field plate and away from the semiconductor material surface. However, the electric field created by the field plate extending over the junction can also inadvertently cause the appearance of an inversion layer in other regions of the p-n junction. In order to avoid this effect, another electrode, typically an annular ring, is deposited on the oxide surface and electrically connected to an additional diffused ring known as a channel stopper. These additional structures further complicate the construction of the device and make more difficult an accurate modeling of the device.